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<p>
This package defines the low-level messages sent between routers,
called the Invisible Internet Network Protocol (I2NP).
</p>
<p>
Not for use by apps, clients or plugins.
</p>
<p>
The Invisible Internet Network Protocol (I2NP) is only a part of how an
application can send messages over the network. The Invisible Internet Client
Protocol (I2CP) defines how client applications written in any language can
communicate with the network routers. In addition, various transport protocols define
the specifics of how data is passed from one router to another over the network. I2NP
does not specify or require any particular transport layer, allowing transport protocols
to work over TCP, Polling HTTP, SMTP+POP3/IMAP, UDP, among anything else
that can pass data. I2NP merely requires that they:</p>
<ul>
<li>
Register a unique identifier for use in RouterAddress structures consisting of no
more than 32 UTF-8 characters.</li>
<li>
Define standard text based options that uniquely define a contact method (for
example .hostname. and .port. or .email address.) as usable in the
RouterAddress structure's set of options.</li>
<li>
Provide a means to reliably deliver a chunk of data, where the contents of any
particular chunk is delivered in order. However, different chunks of data do not
need to be delivered in order.</li>
<li>
Secure the chunks of data from alteration or disclosure (e.g. encrypt them and use
checksums).</li>
<li>
Enable the router to control the transport's bandwidth usage.</li>
<li>
Provide estimates for the latency and bandwidth associated with passing a chunk of
data.</li>
<li>
Provide a programmable interface suitable for integration with various routers.</li>
</ul>
<p>
Transports themselves can implement advanced features, such as steganography,
constant rate delivery, dummy message delivery, and may even run on top of existing
networks, such as mixminion, kazaa, gnunet, and freenet. Transports can even be
written to run over I2P itself, accessing it as a client and mixing the message through
other routers.
</p>
<p>
Sandwiched between I2CP and the various I2P transport protocols, I2NP manages the
routing and mixing of messages between routers, as well as the selection of what
</p>
<h2>Destination Sends a Message</h2>
<p>
Following is a common usage for I2NP Messages.
For other usages, including tunnel building, see the I2NP specification
or the tunnel build document.
</p>
<p>
Whenever a Destination wants to send a message to to another Destination, it
provides its local router with both the Destination structure and the raw bytes of the
message to be sent. The router then determines where to send it, delivers it through
outbound tunnels, instructing the end point to pass it along to the appropriate inbound
tunnel, where it is passed along again to that tunnel's end point and made available to
the target for reception. To understand fully, each step in the process must be
explained in detail.</p>
<ul>
<li>
First, once the originating router receives the message and the Destination, it
attempts to find the LeaseSet associated with it as stored in the Network Database
under the key calculated by SHA256 of the Destination.</li>
<li>
The router then builds a GarlicMessage addressed to the SHA256 of the
PublicKey from the LeaseSet with the real data to be delivered. This
GarlicMessage contains at least one GarlicClove in which there are instructions to
deliver the clove's payload to the Destination. Additional cloves may be present,
and in fact, if the source router desires guaranteed delivery, it will include a clove
requesting source route delivery of a DeliveryStatusMessage back to itself. The
body of the GarlicMessage with all enclosed GarlicCloves is encrypted to the key
specified on the LeaseSet using the ElGamal+AES256 algorithm described in the
data structure spec.</li>
<li>
The router then selects one or more outbound tunnels through which the
GarlicMessage will be delivered.</li>
<li>
Then the router selects one or more of those Lease structures from the LeaseSet
and constructs a TunnelMessage along with DeliveryInstructions for the
outbound tunnel's end point to deliver the GarlicMessage to the inbound tunnel's
gateway router.</li>
<li>
The source router then passes the various TunnelMessages down the outbound
tunnel to that tunnel's end point, where the instructions are decrypted, specifying
where the message should be delivered.</li>
<li>
At this point, the end point must determine how to contact the router specified in
the decrypted DeliveryInstructions, perhaps looking up RouterInfo or
LeaseSet structures in the Network Database, and maybe even delaying a
requested period of time before passing on the message.</li>
<li>
Once the tunnel end point has the data it needs to contact the inbound tunnel's
gateway router, it then attempts to contact it either directly through one of its public
RouterAddress or source routed through one of its published trusted peers. Over
this medium the tunnel end point delivers the GarlicMessage as it was wrapped by
the source router, along with the TunnelId.</li>
<li>
Once delivered to the inbound tunnel's gateway, the gateway builds a
TunnelMessage wrapping the GarlicMessage, encrypting a
DeliveryInstructions to specify local delivery upon arrival at the tunnel's end
point.</li>
<li>
Once the TunnelMessage is passed down to the end point in inbound tunnel, the
router opens the DeliveryInstructions, notes the request to deliver it locally,
and then proceeds to review the contents of the TunnelMessage's payload, which in
this case is a GarlicMessage addressed to the SHA256 of a LeaseSet that it has
published. It then decrypts the payload of the message with ElGamal + AES256.</li>
<li>
After opening up the GarlicMessage, it reviews each of the GarlicCloves and
processes them each. Cloves with DeliveryInstructions addressed to a local
Destination are delivered to the associated client application, other cloves asking
for local processing (e.g. Network Database messages or DeliveryStatusMessages)
are processed, and cloves asking for forwarding to other routers are passed off for
delivery.</li>
</ul>
<p>
There are several important points of note in this scenario. First, the source router
determines how many messages to send, how many outbound tunnels to send them
out, how many inbound tunnels to send them to, and how many cloves should include
DeliveryStatusMessage responses. The algorithm deciding these choices depends
both on the router implementation as well as the Destination's session configuration
options specified to balance the bandwidth, latency, reliability, and anonymity
constraints. Also, instead of using outbound tunnels to get the message to the inbound
tunnel's gateway, the router may decide to source router the message instead. If the
message id for a clove has already been processed or its expiration has passed, the
clove is dropped.
</p>
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